The Restless Tides 



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The horizontal components otthe 

 Moon's force of attraction are least 

 at the points nearest and farthest 

 from the Moon. They are also zero, 

 or very nearly so, around the great 

 circle at right angles to the line 

 joining Sun and Moon because there 

 the attraction forces Fi and F2 are 

 equal and there is no differential 

 force. The tide-raising forces 

 (horizontal attraction forces) are 

 greatest halfway between this and 

 the points nearest and farthest 

 from the Moon, as shown here. 



Water on the Earth's surface is 

 rather like a loose skin on the 

 fairly solid crust of the Earth. If 

 the Moon's force of attraction on 

 the water at a point P on the crust 

 is greater or less than the force 

 of attraction at the center of the 

 more or less rigid Earth, the water 

 tends to move toward or away from 

 the Moon. When P is nearer to the 

 Moon than C the force of attraction 

 F, at P is greater than the force of 

 attraction F2 at C. The differential 

 force F3 (shown in the triangle of 

 forces) acts toward the Moon. When 

 P is on the side of the Earth 

 farther from the Moon F2 is greater 

 than f/, and F3 acts away from the 

 Moon. The differential force is 

 greatest at the points nearest to 

 the Moon and farthest from it, but 

 at these places the Moon's pull is 

 vertically upward, away from the 

 Earth, and since it is only one ten- 

 millionth of the opposite, vertically 

 downward pull of the Earth it does 

 not move the water. It is at points 

 such as P, where the differential 

 force has some horizontal pull along 

 the Earth's surtace, that the Moon's 

 force of attraction becomes a tide- 

 raising force. The Sun exerts a 

 similar tide-raising force, but it 

 is less than half as strong as that 

 of the Moon. Although the Sun is 

 more massive than the Moon it is 

 much farther away, hence its weaker 

 force of attraction. 



Because the Earth rotates around its 

 own axis - the north-south line in 

 the figure - water anywhere on the 

 Earth's surface is pulled first one 

 way and then the other twice every 

 24 hours, 50 minutes. If the Earth 

 were covered entirely by water we 

 should expect as an "equilibrium 

 tide" two permanent "humps" of 

 water on opposite sides of the 

 Earth, as shown here; and the 

 rotation of the Earth would mean 

 that any point P would experience 

 high water at P and again at P' . 

 Unless the Moon happened to be in 

 the plane of the Earth's Equator the 

 two tides at P and P' would not be 

 the same height. At new and full 

 moon, when the Sun and Moon pull in 

 more or less the same direction, the 

 tides are higher and are called 

 spring tides. At the first and third 

 quarters, when the Sun and Moon act 

 at right angles, the tides are less 

 and are called neap tides. 



This diagram (right) shows the rotary 

 tidal current at Nantucket Shoals Light- 

 ship. The lengths of the radiating 

 lines represent the velocity of the 

 current at the beginning of each 

 hour, and their directions indicate 

 the direction of the current. 

 H denotes high water, L low water. 

 The current has a maximum velocity 

 toward the northeast 2% hours before 

 high water, and toward the south- 

 west 272 hours before low water. 



Scale of linoio 



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262 



